Review other people's decks and get advice from a huge community of enthusiasts at tappedout.net. You can upload your own deck and get advice and stats (like pricing estimate). Another similar resource is deckstats.net. Another popular forum is mtgsalvation.com

If you just want to get a quick estimate of one or more cards, you can use mtggoldfish.com (some MTG enthusiasts just collect cards rather than play them)

Once you get a handle on the basics, understand the importance of sideboards